Loft Insulation Advice

littleboo

Hi,

My loft insulation is a bit haphazard and I would like to improve it for obvious reasons. About 40% has Knauf Spaceblanket between the joists which is I think 150 mm, so proud of the 100 mm joists. There is a small area which just has a thin layer of lose material from when the house was built in the 70's, barely anything at all. I also some small areas where I have put 100 mm between the joists and 170 mm perpendicular. (Knauf Glass Mineral Wool) and can extend that to the barely covered area.

So I'm wondering what to do about the area with the Spaceblanket. Do I just lay 100 or 170 mm of Knauf Glass Mineral Wool perpendicular on top of that? Or do I lift the Spaceblanket, put down 100 mm and lay the Spaceflight perpendicular? My loft of very low and long, its horrible to work in, so would like not to lift the Spaceblanket, unless it has to be done.

Green_hopeful

I am not quite sure I follow all the different elements but I have topped up what I have. So long as it’s between the joists and full up I have put more over the top perpendicular as you suggest. In one house where there was space I bought the plastic covered insulation with the foil so I could unroll it and was able to slide it across to where it needed to be. If you have old insulation you just need to make sure it’s between the joists so air can’t get under it. In our current house the professionals had laid it across the joists with a howling wind blowing under it. 

littleboo

My query is about having insulation that is 150mm deep, in 100mm joist, and them adding more insulation at 90 degrees to that. Is that problematic in any way? Normally you would have 100mm between the joists so the second layer was sitting on the first layer and also the joist

grumbler

littleboo said:

So I'm wondering what to do about the area with the Spaceblanket. Do I just lay 100 or 170 mm of Knauf Glass Mineral Wool perpendicular on top of that? Or do I lift the Spaceblanket, put down 100 mm and lay the Spaceflight perpendicular? My loft of very low and long, its horrible to work in, so would like not to lift the Spaceblanket, unless it has to be done.

Obviously, 100 mm between the joists is better, but IMO it won't make a big difference if you put 100 mm on the top.

Do you realise that after you do this it will be really difficult to move in the loft as the joist will be hidden and unpredictable?

FreeBear

The Knauf spaceblanket - Is this the quilted stuff with an outer layer of material to stop the fibres blowing around ?

Even if it isn't, rolling up the old 150mm insulation and replacing with 100mm between the joists is not hugely difficult. You can then lay the 150mm spaceblanket at 90 degrees. The difference between having 250mm, 270mm, or 300mm of insulation is not going to make any significant difference. More is better, but it is also a case of diminishing returns.

You don't really want any gaps between the layers of insulation where wind can blow through.

alleycat`

Yes you can do what you want to do.

Fill the joist gaps where it's rubbish before putting anything running cross ways.

I'd put the newer better stuff below the old stuff (personal choice) but if it's that bad to work in I'd just cover it as you go.

Make sure you wear a mask and appropriate covering (as it's not pleasant to mess with).

Caveats are:-

Don't cover electrical wiring unless you know how to "de-rate" them.

Be aware you won't be able to see the joists, which makes going in and out of the loft a total pain (f you need to access it regularly).

Check there's no damp and, depending on how your loft is "aired" for want of a better term, don't block the ventilation with the insulation or you'll cause yourself problems down the road.

FreeBear

alleycat` said: Caveats are:-

Don't cover electrical wiring unless you know how to "de-rate" them.

Be aware you won't be able to see the joists, which makes going in and out of the loft a total pain (f you need to access it regularly).

Wiring in a loft is generally going to be lighting - With LED bulbs everywhere, even 1mm² cable is going to be well overrated. In a plasterboard ceiling, covered by more than 100mm of insulation, 1mm² cable is good for 10A. A 7W LED lamp will draw ~0.03A.

If your ring main loops around in the loft, then yes, you do need to be a little more cautious. 2.5mm² cable under the same conditions is rated at 17A max, but then how often would someone be running 4KW loads on a domestic ring.

Loft legs and boarding is a sensible answer if you want/need access or storage in the loft.

alleycat`

FreeBear said:

alleycat` said: Caveats are:-

Don't cover electrical wiring unless you know how to "de-rate" them.

Be aware you won't be able to see the joists, which makes going in and out of the loft a total pain (f you need to access it regularly).

Wiring in a loft is generally going to be lighting - With LED bulbs everywhere, even 1mm² cable is going to be well overrated. In a plasterboard ceiling, covered by more than 100mm of insulation, 1mm² cable is good for 10A. A 7W LED lamp will draw ~0.03A.

If your ring main loops around in the loft, then yes, you do need to be a little more cautious. 2.5mm² cable under the same conditions is rated at 17A max, but then how often would someone be running 4KW loads on a domestic ring.

Loft legs and boarding is a sensible answer if you want/need access or storage in the loft.

I found 10mm T&E for an electric shower in my loft buried under insulation which is why i mentioned it.

As you say it's an unusual / outlier scenario but mentioned for completeness.

Apodemus

alleycat` said:

FreeBear said:

alleycat` said: Caveats are:-

Don't cover electrical wiring unless you know how to "de-rate" them.

Be aware you won't be able to see the joists, which makes going in and out of the loft a total pain (f you need to access it regularly).

Wiring in a loft is generally going to be lighting - With LED bulbs everywhere, even 1mm² cable is going to be well overrated. In a plasterboard ceiling, covered by more than 100mm of insulation, 1mm² cable is good for 10A. A 7W LED lamp will draw ~0.03A.

If your ring main loops around in the loft, then yes, you do need to be a little more cautious. 2.5mm² cable under the same conditions is rated at 17A max, but then how often would someone be running 4KW loads on a domestic ring.

Loft legs and boarding is a sensible answer if you want/need access or storage in the loft.

I found 10mm T&E for an electric shower in my loft buried under insulation which is why i mentioned it.

As you say it's an unusual / outlier scenario but mentioned for completeness.

Yes, in a single-storey house, by far the easiest way for it to be wired is through the loft-space, so all sorts of horrors can be up there!

littleboo

Thanks for all the replies. As Freebear suggests, I'll look to lift the 150mm and replace it with 100mm, with the 150mm on top at 90 degrees. There will be a lot of cursing along the way